The motivation for the transmission of hi-fi audio signals is an outgrowth of the need to transmit the signals from each entertainment source to every corresponding receiver in a building. (Hereinafter, the term "residence" will be used to generally refer to any building that contains telephone wiring, such as an ordinary single-family home, an apartment, or a commercial building.) There is a need, for example, to transmit the signals from video entertainment sources, i.e. video cassette recorders (VCRs), cable converters, and satellite signal receivers, to every video receiver, i.e., each television. A similar need for communication between audio sources and receivers also exists. In audio systems, the sources include cassette decks, record players, compact disc (CD) players, FM tuners, and turntables. The receivers are the loudspeakers and earphones while amplifiers can be classified as part of either category.
In the classical situation, source and receiver are located close to each other in the same room. To enjoy music or video, however, one does not need to be in close proximity to (or even in the same room as) the signal source. Rather, one only needs to be within visual range of the video receiver or audio range of the audio receiver (so as to be able to see or hear the desired signals) and have an ability to control the sources. Thus, an ability to communicate audio, video, and control signals between rooms will allow one to enjoy music and video using only speakers and a television. While the parent application provided a solution to the problem of transmission of video and control signals, no inexpensive solution to the transmission of hi-fi audio, much less the simultaneous transmission of all three signals, has been developed to date.
Hi-fi audio signals can be transmitted across a residence using radio waves as well as by transmission across a wire or other conductive path. Broadcasting signals, however, allows for the possibility of unintended reception outside the residence, and also allows for the possibility of interference from external sources broadcasting at the same frequency. Government regulations covering the broadcast of these signals also present significant obstacles.
Common conductive paths within a residence include power wiring (i.e., wiring that carries 120 VAC, 60 Hz household power), telephone wiring, and coaxial cable. Coaxial cable does not offer a comprehensive solution because it is not available in most residences. It is also bulky, stiff, and unwieldy. Moreover, the signal splitters commonly used in coaxial cabling block transmission between the two downstream ports (i.e., the output ports of the splitters), preventing communication across some of the conductive paths.
Transmission across power wiring is difficult because electrical appliances can impart significant noise onto the wiring network, and because the conductive path is often broken across a fuse box or circuit breaker. Although some systems use power wiring as a conductive path for hi-fi transmission, the systems are relatively expensive, owing to the need for overcoming extremely high noise on the power lines. Even using expensive circuitry, line noise may be so high that it cannot be suppressed in many situations. Moreover, such systems cannot reliably transmit between the differently phased conductors on a 120V residential system, because the conductive paths used by the two phases may only connect far from the residence. Finally, video signals typically cannot be transmitted over power lines with any reasonable degree of quality, so simultaneous transmission of television signals and audio is not possible.
Telephone wiring also presents significant difficulties to the transmission of audio signals. Two obvious difficulties are the requirement of avoiding interference with telephone communications and conforming with all regulations that govern devices that connect to the public telephone network. Other difficulties are presented by the transmission properties of telephone wiring. These include the attenuation of the telephone wiring itself, the attenuation caused by junctions in the wiring and connected telephones that drain RF energy from the network, and switching devices that break conductive paths.
Devices are available that overcome some of these difficulties to achieve transmission of intelligible audio. Radio Shack.TM., for example, manufactures a telephone that, when used with an identical cooperating telephone, provides intercom communication at frequencies above the voiceband. These phones work over ordinary telephone lines used in residences. In addition to its limitation as a monaural rather than a stereo signal, however, the sound quality produced by this telephone does not approach that of most hi-fi systems. That is, such a system cannot transmit high fidelity sounds between the telephones in a manner that would maintain the high fidelity at the receiving telephone. The same is true for other systems known to transmit audio information across active telephone wiring. None of these systems, moreover, simultaneously transmit video signals or control signals from infrared transmitters.
While transmission of digital signals within a residence is not currently an urgent need, that situation is expected to change rapidly over the next several years. There are currently several systems, designed to be used in office environments, that perform digital transmission over active telephone wires. The catalog of the Black Box Corporation, for example, includes several transmit/receive pairs that perform this function. These devices connect to a digital device to derive a digital datastream that adheres to a particular format, e.g., the IEEE RS-232 standard. This information is converted to a time varying voltage at frequencies above the voiceband of telephones. These signals are then fed to an active telephone wire (i.e., a wire used for voiceband communication) that connects directly from point A to point B without any devices connected in the middle (a so-called "point-to-point" connection). This line typically connects between a telephone and a PBX device. At the end of the line, a receiving device connects to detect the high frequency voltage variations, and convert them back to the original digital datastream.
Because this system transmits data over a point-to-point telephone lines that do not include splits, branches, or telephone devices that are connected in the middle, they may not work over networks with arbitrary topologies and telephone devices connected at random points, features found in the internal telephone wiring of nearly all residences.